= Solution
{c}
The <Weiss temperature> $\Theta$ is the extrapolated zero of $1/\chi$ in a <Curie–Weiss law> fit. It summarizes the leading high-<temperature> effect of interactions between <magnetic moments>. For the sign convention
$$
H_{\rm ex}=-\sum_{i<j}\mathcal J_{ij}\,\mathbf J_i\cdot\mathbf J_j,
$$
where $\mathbf J_i$ are dimensionless <angular momentum operators>, <mean-field theory> gives
$$
\Theta=\frac{J(J+1)}{3k_B}\sum_j\mathcal J_{ij}.
$$
Thus positive net <exchange interaction> in this convention favours parallel <magnetic moments>, giving $\Theta>0$ and tendencies towards <ferromagnetism>; negative net <exchange interaction> favours antiparallel <magnetic moments>, giving $\Theta<0$ and <antiferromagnetic> tendencies. Competing interactions can partially cancel in this sum. The <Weiss temperature> need not equal the actual ordering <temperature>, and its sign alone is not a complete determination of the low-<temperature> ordered structure.
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